This invention relates generally to holographic elements, and, more particularly, to the manufacture of holographic elements for use with fiber and integrated optic systems.
Holography is a technique for recording, and later reconstructing, the amplitude and phase distribution of a coherent wave disturbance. Generally, the technique utilized for providing a hologram is accomplished by recording the pattern of interference between an unknown wave of interest and a known wave. These waves, one for example, reflected from an object, usually called the object wave, and the other which bypasses the object, usually called the reference wave, strike a light-sensitive recording medium, such as a photographic film or plate. Thus, incident on the recording medium is the sum of the light from the object and the mutually coherent reference wave.
While all light-sensitive recording media respond only to light intensity, none the less in the pattern of interference between the two waves there is preserved a complete record of the amplitude and the phase distribution of one of the waves with respect to the other. Amplitude information is preserved as a modulation of the depth of the interference fringes, while phase information is preserved as variations of the positions of the fringes.
The photographic recording obtained is known as a hologram although this record generally bears no resemblance to the original object, but rather is a collection of many fine fringes which appear in rather irregular patterns. None the less, when this photographic transparency (hologram) is illuminated by coherent light corresponding to the reference wave one of the transmitted wave components is an exact duplication of the original object wave.
Recently great use has been made of holographic optical elements for use in fiber optic communications links and integrated optics systems. Examples of such holographic optical elements (holograms) for use in fiber optic communication links and integrated optic systems can be found in the form of multiplexer/demultiplexers of the type disclosed in the inventors' U.S. patent application Ser. No. 144,819 (now U.S. Pat. No. 4,359,259) filed Apr. 29, 1980, in couplers to waveguides such as set forth in U.S. Pat. No. 3,885,856 issued May 27, 1975. In all of the above applications an optical fiber is utilized with one of the beams in order to produce the desired hologram. Unfortunately, the utilization of an optical fiber within the holographic manufacturing technique produces a so-called speckle pattern, which is due to a random distribution of intensities and polarization. Since the interference phenomenon is sensitive to both these parameters, there will be substantial areas on the hologram where a poor interference pattern is recorded. Consequently, the overall efficiency of the system is considerably reduced.